The invention relates to an electrostatic speaker, specifically a class D electro-static micro-speaker.
Speakers produce sound waves by converting electrical signals into air pressure pulses. A classic dynamic loudspeaker uses a voice coil in a magnetic gap to move a cone and convert the electrical signals into air pressure pulses. A prior-art electrostatic speaker 100, such as shown in FIG. 1, has a diaphragm 105 biased (by springs 110) at a midpoint within a usable gap 115. An analog electric signal 120 (i.e., an audio signal) is applied to the diaphragm 105 which cyclically draws the diaphragm 105 toward from a back-plate 125 and allows the diaphragm 105 to return to a resting position (by the biasing force of the springs 110).
In the prior-art electrostatic speaker 100, the springs 110, along with air that is moved (e.g., in the usable gap 115), tend to dampen the response of the electrostatic speaker 100. The electrostatic speaker 100 also has numerous nonlinear mechanisms (e.g., mechanical stiffness which varies based on the position of the diaphragm 105) which affect the performance of the speaker 100. In addition, the diaphragm is limited to only about 10% of its actual range to reduce linearity issues and to guard against “snap-in” (i.e., where the diaphragm latches in a fully extended position). Micro-speakers are a sub-set of electrostatic speakers, referring generally to the size of the electrostatic speaker. Micro-speakers are small speakers such as those found in cell phones.
Class D amplifiers combine a desired output signal with a relatively high-frequency signal to generate a digital signal which can be amplified by switching power devices. The frequency of the high-frequency signal is typically chosen to be ten or more times the highest frequency of interest in the desired output signal. The digital signal includes both a low-frequency component (i.e., the desired output signal) and a high-frequency component. A passive low-pass filter is used to remove the high-frequency component, and recover the desired low-frequency output signal.